Show how your question is different from what you've already gone over.
It must be different, or you wouldn't be asking again, right? So something like:
I know you said it must be red and green, but then your documentation says it must be friendly for colorblind people. These two things are at odds, so which do you want me to do?
The documentation says the page must load "very fast", but I'm not sure specifically what that means. Is one second very fast? One tenth of a second?
If you go to them and simply ask what colors should you make the page or how quickly it should load, then they're not going to understand why you're asking about it again. If you explicitly acknowledge the information you've already been given and why it's insufficient, then there's no way they can retort with "go read the documentation." If you say "I'm not sure what colors to use", then of course they'll say "I already told you this. Why are you asking me again?"
Another thing that can help is to put more effort into seeing these problems ahead of time, so you don't have to repeatedly go back to them for clarification. If you ask them a question about the color scheme ten different times, then it's completely understandable they'd get frustrated, even if they're all legitimate. Think through the request thoroughly enough the first time that you don't need to keep going back to them to clarify things.
Let me give you a real example from today that's not software related. My boss left a couple papers on my desk that I needed, but I was working elsewhere. I texted someone in that office to grab them for me because I was going to see them later. They were the only two papers on or in my desk, and my desk is very clean in general. Here's how the conversation went:
Me: Hi, will you grab the papers on my desk and bring them to me later?
[A couple minutes pass]
Them: Where are they?
Me: On my desk. (Thinking: didn't my last text say that?)
[A couple more minutes pass]
Them: Is it the X papers?
Me: Yes. (Thinking: those are literally the only papers I could
possibly mean. Right?) Are there any other papers on my desk?
Them: No, just those two.
At this point, I'm wondering if they drank too much the night before or hit their head or something. From my perspective, they kept asking for clarification when I had clearly given them enough information to do the job. They never justified their confusion. I said "grab papers on my desk". All they had to was look at my desk and grab the only visible papers.
Now imagine if they had sent the following along with their initial response: "I don't want the people sitting next to you thinking I'm stealing things from your desk. Can you tell me exactly where the papers are and what they look like so I can grab them confidently and not seem suspicious?" Then I would have understood why they were asking and could've promptly resolved it. As it was, I'm sure we both ended up annoyed.
In conclusion, make your question specific, explicitly give a reason for your confusion and why you can't solve it on your own, and do your very best to ask all your questions when the problem first arises.